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By Xinzhi (Thomas) Du

"In order to monitor the overburden strata deformation over a longwall mining area in the Pittsburgh Coal Seam, three boreholes were drilled across the longwall panel. Each borehole employed multiple-point borehole extensometers (MBE) to monitor overburden strata movement during and after the longwall face passed under the study area. The longwall face in the study area was 1,428 ft wide and 4,000 ft long. The average mining height was about 7 ft. The overburden depth ranged from 550 ft to 650 ft with an average of 600 ft. The final extensometer readings indicated that overburden strata initially showed sign of movement when the longwall face was 700 ft inby the MBE. The rate of strata movement accelerated when the face was 300 ft inby MBE. When the face was directly under MBE, a sudden anchor displacement occurred, indicating rapid strata movement once under-mined gob. The readings showed insignificant strata movement after the longwall face was 400–700 ft outby MBE.INTRODUCTIONThe longwall mining method is an efficient mining method of underground mining. Surface and subsurface strata are inevitably disturbed above the longwall mined out gob. Figure 1 shows the four zones of disturbance in the overburden strata in response to longwall mining.A longwall shear cuts the coal seam and allows the immediate roof strata to cave into irregular blocky material to fill the void. This is called the caved zone. The caved zone extends from the mining horizon to six or eight times the mining height, depending on the bulk factor of each caving strata layer. The fractured zone is located above the caved zone, in which the strata breaks into regular blocks by vertical or sub-vertical fractures caused by bending forces due to the weight of the underlying strata. The surface soil and rock crack zone extends to approximately 50 ft below the surface. The surface cracking is caused by downward movement of the surface. Between the fractured zone and surface crack zone is the continuous deformation zone. The strata in this zone deforms gently without causing any major cracks that extend long enough to cut through the thickness of the strata, as in the fractured zone (Peng, 2006).The objective of this project is to provide fundamental rock mechanics research about overburden strata movement response to longwall-related activity. The findings aid in better understanding pillar stability, gas well subsidence management, hydraulic impacts, and numerical modeling."

Jan 1, 2018

By Paul C. Hagan, Jianhang Chen, Serkan Saydam

"Modified cable bolts are commonly used in underground mines due to their superior performance in preventing bed separation when compared with plain strands. To better test the axial performance of a wide range of cable bolts, a new Laboratory Short Encapsulation Pull Test (LSEPT) facility was developed. The facility simulates the interaction between cable bolts and surrounding rock mass, using artificial rock cylinders with a diameter of 300 mm in which the cable bolt is grouted. Furthermore, the joint where the load is applied is left unconstrained to allow shear slippage at the cable/grout or grout/ rock interface. Based on this apparatus, a series of pull tests were undertaken using the MW9 modified bulb cable bolt. Various parameters including embedment length, test material strength and borehole size were evaluated. It was found that within a limited range of 360 mm, there is a linear relationship between the maximum bearing capacity of the cable bolt and embedment length. Beyond 360 mm, the peak capacity continues to rise but with a much lower slope. When the MW9 cable bolt was grouted in a weak test material, failure always took place along the grout/ rock interface. Interestingly, increasing the borehole diameter from 42 mm to 52 mm in weak test material altered the failure mode from grout/rock interface to cable/grout interface and improved the performance in terms of both peak and residual capacity.INTRODUCTIONFully grouted cable bolts have been used in underground mining industry for many decades. Initially, plain cable bolts manufactured by winding seven flexible steel wires together were commonly used (Thorne and Muller, 1964). Laboratory and field tests lead to a better understanding of the transfer process between grouted cable bolts and the surrounding rock mass, which is beneficial to improve the reinforcing performance of cable bolts in engineering practice (Fuller and Cox, 1975; Cox and Fuller, 1977; Schmuck, 1979; Stillborg, 1984). Nevertheless, as in many cases the initial cable was sourced from discarded steel with a smooth surface along the strand, plain cable bolts did not always meet performance expectations (Windsor, 1992). Over time, it was realised that cable surface geometry plays an important role in determining anchorage performance. Consequently, modified forms of cable bolts were developed. For instance, twin-strands which were made by using spacers to combine two plain cables together improved anchorage capacity two-fold (Matthews, Tillmann and Worotnicki, 1983). However, this required a large borehole to be drilled. Epoxycoated strands had remarkable performance in practice but were difficult to install without damaging the epoxy layer (Dorsten, Hunt and Kent, 1984). In the 1990s, there was a rapid development in the cable bolt design. A typical example was birdcaged cable bolts. They were initially suggested by Hutchins et al. (1990) via unwrapping traditional seven-wire strands and generating a number of nodes together with antinodes along the strand. But it was difficult to make birdcaged cable bolts that could be coiled for transporting."

Jan 1, 2015

By Ken Barish

Jane Mine of the Keystone Coal Mining Corporation was started in 1962 and is in the Lower Freeport seas located in Armstrong County, Pennsylvania, approximately 45 miles northeast of Pittsburgh. The mine is a slope/shaft complex with the cover ranging from 100 feet at the slope to 450 feet near the back end of the property. The seam height ranges from 48 to 72 inches. A variety of roof conditions have been en¬countered, and an equal variety of roof control systems have been tried with varying degrees of success. Initially, places could be mined as deep as 100 feet without any permanent roof support. After several problems and the advent of the Health and Safety Act of 1969, roof control became more and more prevalent and predominant at the Jane Mine. It started with 4 foot conventional oolts. As the mine progressed in the northeasterly direction, roof conditions changed from the extremely good roof that was encountered initially, to bad roof; and other types of roof support were required. Sixty pound rails and 6 inch 25 pound to the foot steel I-beams were used for supplemental support in long life headings such as belt and track. Resin bolting was done in many areas for long life support due to the deterioration of the shale roof over time. Several areas had to be abandoned because of uncontrollable top.

Jan 1, 1984

By Anton Sroka

The German hard coal production predominantly derives from underground multiseam mining activities. Beside common surface damages, ?inner mine damages? on underground infrastructures (i.e. main roads, cross cuts and shafts) due to mining excavations occur to an increasing extent. As these "inner mine damages" dramatically increased because of the development towards high performance longwall mining technologies, this matter raised to be a severe problem concerning mining economics. To protect durable and long lasting underground infrastructures, precautionary measures such as suitable mine layouts, adapted planning of mining operations and moderate face advances have to be considered. Some examples of "inner mine damages" recorded by means of mine surveying methods are given in this paper. An empirical-mathematical model based on the principles of mining subsidence engineering methods will be introduced. This model allows to minimize ?inner mine damages? and helps to support and optimize mining processes.

Jan 1, 1999

By Kot F. Unrug

The shale response to moisture changes and the major finding of the appropriate research concerning this subject are briefly reported in this paper. Also described are mine ambient atmospheric conditions which are responsible for moisture changes in mines. The need for a quantitative measure of the weathering potential of roof. rock is demonstrated in relation to its influence on safety and the potential for increasing the cost of mine operations. The weatherability test presented in this paper has been developed and used in many mines. It imitates the natural process in a mine by subjecting the rock sample to alternating wet and dry cycles. The natural changes are thus accelerated, but are similar to the natural conditions to which the surrounding excavation rock will be exposed. The measure of the weatherability index is the percentage of the initial sample mass remaining after the test as the largest fragment of the sample. Thus, the index changes, from 100% for rock affected by moisture changes which disintegrates completely, to 0% for rock totally unaffected. In this paper, the apparatus and testing procedures are described. Also presented are examples of test results. with a discussion of the design implication to he considered by mine planners.

Jan 1, 1997

By R. Quentin Eades, Kyle Perry

"Surface mines continue to implement highwalls for several reasons, such as increasing recovery, improving margins, and justifying higher stripping ratios. Highwall stability is a complex issue that is dependent upon a variety of mining and geologic factors, and a safe design is necessary for a successful surface operation. To improve highwall stability, it is important to understand the connection between local geology and blasting.Explosives are employed throughout the mining industry for primary rock breakage. There are a number of controlled blasting techniques that can be implemented to improve highwall stability. These include line drilling, smooth wall blasting, trim blasting, buffer blasting, air decking, and presplitting. Each of these techniques have associated advantages and disadvantages. Understanding local geology is necessary for selecting the appropriate controlled blasting technique. Furthermore, understanding the limitations and conditions for successful implementation of each technique is necessary. A discussion of the impact of geologic conditions on highwall stability is provided. Additionally, discussion is provided for the successful incorporation of the controlled blasting techniques listed above, and the associated mining and geologic factors that influence the selection and design of controlled blasting plans. Finally, a new methodology is proposed.INTRODUCTIONExplosives are used throughout the mining industry as the standard for primary rock breakage, a critical part of the mining cycle. In 2015, the U.S.. consumed 2.2 million tons of explosives (Apodaca, 2015). The coal mining industry accounted for the majority of explosive consumption, accounting for approximately 63% of total explosives. The quarrying and nonmetal mining industries accounted for 12% of explosive consumption, and the metal mining industry accounted for 9% (Apodaca, 2015). This constitutes 84% of the explosives used in the U.S. However, the energy released during the detonation process is often in excess of that required to adequately fragment the surrounding rock (Jhanwar and Jethwa, 2000). This excessive energy, along with over confinement and poor blast geometry, will cause damage to the undisturbed rock mass beyond the intended boundary of the blast. This event is known as overbreak or back-break (ISEE, 2011)."

Jan 1, 2018

By Stephen P. Signer

A method to assist in the evaluation and selection of roof bolts using in situ measurements of roof bolt loading has been developed by researchers of the Spokane Research Center, National Institute of Occupational Safety and Health. Both axial and bending forces are measured by strain gauges at many locations along the length of fully grouted bolts during various stages of mining. This information is then used to ( I) predict bolt loading for variations in bolt spacing, grade, and diameter, (2) calculate the design changes necessary in zones where bending loads are high, and (3) determine if bolt length is adequate. Results from several case studies of full-column bolt loading in coal mine gate roads are used to illustrate the design method. This knowledge will give design engineers a tool for the selection of roof support systems that will improve underground safety by reducing roof bolt failures.

Jan 1, 1997

By A. Chrzanowski

The integrated approach to monitoring surveys is based on a simultaneous utilization of geodetic, photogrammetric and geotechnical measurements of displacements, tilts, strains and any other geometrical parameters of deformation. New techniques for the integrated surveys are being developed at the University of New Brunswick in cooperation with the Canada Centre for Mineral and Energy Technology. A telemetry system for continuous monitoring of geotechnical measurements under difficult climatic and topographic conditions has been constructed and successfully tested in a coal mining area in western Canada. A generalized method for analysing the integrated deformation measurements has been developed and applied in the test area to yield three dimensional empirical modelling of surface movements from geodetic, photogrammetric and tilt surveys. The most appropriate model can be chosen from a statistical assessment of several modelling possibilities. The empirical model of the deformation can be employed in a "calibration" of the prediction (deterministic) model of the ground movements. A computer programme for two- and three-dimensional finite element modelling of elasto-plastic deformations has been developed at UNB as a part of the integrated approach. The :EM has been tested in predicting deformation in the test area in difficult geological conditions which were associated with an extraction of a steeply inclined coal seam. The results gave good agreement with the actual displacements obtained from geodetic measurements and confirmed the existence of a suspected fault.

Jan 1, 1984

By Jialin Xu

Bed separations position, size, development characteristics and their influence factors have been analyzed in-depth by experimental and theoretical methods. The results show that the key strata determine the creation, development and space-time distribution of the overburden bed separations and fractures. Based on the dynamic growth principle during the two-stage development of bed separations, the theory of section-grouting for the overburden bed separation space and the O-ring theory for the pressure-relieved methane drainage are proposed. The technique has been applied in some coal mines in China to reduce surface subsidence and for methane drainage. Keywords: rock strata movements; key stratum; bed separation; bed separation grouting; pressure-relieved methane drainage

Jan 1, 2003

By Lorraine Kent

Mine tunnel support using rockbolts was introduced in UK coal mines over the period 1987- 1992 and is now used in over 70% of all mine tunnels. In every case the reinforcement design has been based on the observational approach. After the initial reinforcement design has been established appropriate action levels are defined for predetermined amounts of movement indicating the onset of instability. If the action levels are exceeded appropriate remedial action is taken usually in the form of additional support or reinforcement to maintain stability. This paper outlines the observational design method as used in UK coal mine tunnels. It discusses the low cost instrumentation which has been developed to monitor roof deformation, dual height telltales, how action levels are defined based on the geotechnical environment and the types of remedial measures typically taken. An alternative approach for dealing with areas of increased deformation involves reacting to early roof movement trends and modifying the support installed at the face of the development accordingly. Installation of additional support at the face of the heading improves ground control in poor geotechnical conditions and has the advantage of potentially reducing later additional support costs. A case study is described to illustrate how 'early' action levels can be incorporated into a roof support plan enabling a flexible roof support strategy. Such a system will be site specific and continued analysis of monitored information will be required to confirm the action levels or to adjust them according to any change in geotechnical environment. More accurate roof deformation monitoring systems, such as the Remote Reading Telltale system recently developed by Rock Mechanics Technology Ltd. (RMT) and currently undergoing Mines Safety and Health Administration (MSHA) approval, are ideally suited to this application. The Remote Reading Telltale system provides accurate mine wide monitoring of roof condition on a surface PC.

Jan 1, 1999

By D. W. Evans

Mine subsidence problems due to coal extraction have occurred in a number of areas throughout the United States. Depending on the local geology, the depth of the mined seam, the type of mining method employed for extraction, and other related factors, subsidence can result in anywhere from slight displacements to a total loss of ground. These conditions can have serious consequences on the health and safety of the people living above the mined areas. A number of methods exist to provide support for the ground overlying mines. All of these methods employ some mechanism for providing additional support to either the roof of the mine or the sides of the pillars. Since failure usually occurs due to overburden stresses which are transmitted to the roof and pillars, these methods employ techniques to counteract stresses induced due to the coal extraction. The applicability of any particular method will depend on the availability of backfill material, the areal extent of the affected area, and of course, the cost. The primary use of coal ash for subsidence control has been as a backfill material for the stabilization of large areas. Large scale backfilling projects for subsidence control have been performed in Rock Springs, Wyoming, Wilkes-Barre and Scranton, Pennsylvania, and in Fairmont, West Virginia. Some of these projects have utilized sand, as well as, coal ash as backfill material. Much of the backfilling work performed in the United States has been done in an effort to control mine subsidence in abandoned mines. South Africa, on the other hand, has used backfilling extensively both for ash disposal and to increase extraction in active mines. The utilization of repetitive fill and mine techniques has resulted in extraction increases of as much as eight percent in shallow mines and as much as twelve percent in deep mines.(1) The coal ash injection system which can be used for backfilling depends largely upon whether the mine voids are dry or submerged. For dry mines, hydraulic or pneumatic conveyance systems can be used, while submerged mines are limited to backfilling by hydraulic methods. The use of coal ash as a backfill material has a number of advantages, such as: 1. Coal ash is available in areas throughout the United States and in many instances is conveniently located near areas requiring backfilling. 2. Coal ash is available in significant quantities since many coals when burned will result in about 15 to 30 percent ash by weight. 3. Coal ash as backfill is, in many instances, the most economical alternative fill material. Due to the costs associated with above ground disposal, power companies will often times give the material away to lower their overall operating expenses. 4. Coal ash is usually pozzolanic in nature, that is, this material results in cementitious bonding within the backfilled matrix of material.

Jan 1, 1982

By Deno Pappas

During the past few years, the Pittsburgh Research Laboratory of the National Institute for Occupational Safety and Health (NIOSH) has been evaluating developments in sealant support systems from a ground control perspective. The proper selection and use of a sealant material can significantly enhance ground control which may result in a decrease in worker related injuries due to ground falls. The purpose of this paper is threefold: to briefly review the current state¬of-the-art technology in shotcrete and membrane developments, to evaluate the preliminary findings of a long-terra underground study of various types of sealant materials, and to examine installation practices that are critical for an effective sealant material. The underground study utilizes NIOSH's Lake Lynn Laboratory Experimental Mine to evaluate the long-term performance of several types of shotcrete and membrane materials. Sealant performance to date have been evaluated on a regular basis over a two year period. Although the study is still ongoing, critical mining practices were identified that may seriously effect the bond of the sealant materials to the mine roof and rib; most notably, the importance of scaling and thorough cleaning of the rib prior to application. Also, results from an extensive series of Schmidt Hammer tests found that the shoterete increased in strength by 70% during the humid summer months.

Jan 1, 2002

By John M. Karhnak

The Bureau of Mines has a long history of research in Ground Control. For many years, this work was done at Bureau facilities by Bureau researchers. As the tiles changed, however, the area of research interest broadened, and additional methods of implementation, such as con- tracts, were added. 'Ibis paper outlines the organization, goals, and methods of implementation of the Bureau Ground Control Program, along with procedures for project selection. This program organization is followed by several specific examples of Bureau research that will be soon ready for use in the mines.

Jan 1, 1981

By Murali Gadde

Among different types of ground control problems associated with underground coal mining, those related to in situ stresses are the most common ones affecting the safety and economy of a mining operation. As a result, this has been the focus of many ground control research works done in the last three decades. This paper summarizes a recent work done on in situ stress related ground control issues of underground coal mine development workings. The approach used in this study was three¬-dimensional finite element modeling by an implicit code, ABAQUS/Standard. In the models, roof stability evaluations were made using Hoek-Brown rock mass failure criterion. Influence of the in situ maximum horizontal stress angle on the stability of both entry and intersection was examined to help determine the best development layout orientation. Also, the effect of in situ stress ratios on the stability of development openings was studied. To make the work more general, both 'low' and 'high' in situ stress fields were considered In general, the modeling results indicated that entries oriented in the direction of maximum horizontal stress were in the best condition while those oriented at 90° had the least stability. For intersections, the maximum and the minimum stability were seen at 0°/90° and 45°, respectively. However, under some combinations of input conditions, the best or the worst conditions were noticed at same other orientations as well. Based on these findings, layout design charts were prepared for both entries and intersections. It was also found that the change in ground conditions with change in layout orientation was more significant for 'low' in situ stress fields than for the higher ones. Change in the average safety factor with change in the ratio of in situ maximum horizontal to vertical stress resembled a lognormal distribution curve. However, the effect of the ratio of in situ maximum to minimum horizontal stress lacked such a clear trend for different input combinations considered.

Jan 1, 2004

By Ken Mills

Dartbrook Mine (Australia) works the Wynn seam, which has a moderate propensity for spontaneous combustion. Prior to any mining a commitment was made to place a segregation barrier pillar between longwall blocks 4 and 5. This was to segregate goaves and improve management controls in the event of a spontaneous combustion event within previous longwall goaves. An alternative to the segregation barrier known as a Consolidated Barrier was developed; firstly as a concept, then computer modelled and finally field tested. It consists of a wider than normal chain pillar with concrete Mega-Seals placed in cut-throughs. Permeability and stress measurements were conducted in the surrounding coal of the seal. Remote monitoring took place as the longwall face approached and then passed by the seal. Results indicate that as the longwall retreats past the seal, horizontal stress increases. The additional stress leads to an increase in confinement and a subsequent reduction in permeability.

Jan 1, 1999

By J. R. Stoltz

To date, there has been limited formal research dealing with the use of Mobile Roof Supports (MRS) for pillar recovery. This paper, which is a portion of a larger project, presents convergence data from 4 different mine case studies. MULSIM was used to create a numerical model. The resulting information from this model was then compared to the actual measured values. MULSIM was also employed to analyze the industry-accepted pillar/barrier layout for mining underground reserves with the Joy Highwall Mining System. This layout was examined for both a low cover and a high cover extreme,

Jan 1, 1999

By Eric Westman

Ground control problems caused by multiple-seam interaction frequently sterilize coal reserves. Interaction mechanisms differ for overmining and undermining and are controlled by mining, spatial. and geologic conditions at each site. A multiple-seam interaction analysis software program, UGLY (Uppersearn Gateroad Longwall Stability), has been used to evaluate sterilization of coal reserves due to overmining at three locations. Four case histories were reviewed to determine the effectiveness of improved ground control in mitigating interaction effects.

Jan 1, 1997

By A. Grenoble

The mining of seams in close proximity can greatly accentuate interaction problems. At distances of less than 110 feet vertically interaction can occur for both over and under mining. Research into ground control problems resulting from undermining a previously mined seam has been carried out using finite element and body-loaded photoelastic modeling methods in conjunction with numerous case studies. Analysis of field data was used to validate and expand the model results. Research findings demonstrate situations under which the worst ground control conditions can be expected to occur for a specified geologic environment, depth, innerburden spacing and mining geometry. Special emphasis has been placed on the effects of pillar load transfer on the lower seam in terms of pillar and floor stability and roof control. Roof and floor condition indexes are used to summarize structural and lithologic conditions in the affected seam. The effects of joint orientation and continuity are incorporated in the analysis and the research results have been compiled into a software package for application by the field engineer.

Jan 1, 1984

By John A. Rusnak

Point load testing is used to determine rock strength indexes in geotechnical practice. The point load test apparatus and procedure enables economical testing of core or lump rock samples in either a field or laboratory setting. In order to estimate uniaxial compressive strength, index-to-strength conversion factors are used. These factors have been proposed by various researchers and are dependent upon rock type. This study involved the extensive load France and point load testing of coal measure rocks in six states. More than 10.000 individual test results. From 908 distinct rock units, sere used in the study. Rock lithologies were classified into general categories and conversion factors were determined For each category. This allows for intact rock strength data to he made available through point load testing for numerical geotechnical analysis and empirical rock mass classification systems such as the Coal Mine Roof Rating (CMRR).

Jan 1, 2000

By C. D. Peters

The Bureau of Mines has been researching remote sensing analysis of an underground coal mining area in Utah and computer-aided methods of defining ground control hazards in this mining area. As a result of these studies, a preliminary potential hazards map was developed for the test site. This map is based primarily upon a lineament (linear feature) analysis of Landsat satellite data. Other data used were paleochannel location data, resistivity data, and information from surface and underground field checking of the lineaments and ground control problems in the mine. Lineaments proved to be reliable indicators of hazardous areas, although the type and degree of hazard varies with different lineament trends and the presence of other conditions such as lineament intersections and intersections with other geologic structures or with paleochannels. Lineaments were observed to match faults, joints, and/or joint trends in the area. The paleochannel data, derived from drill hole data, provided basic information about potentially hazardous areas in the mine by suggesting where paleochannels could cause instability in the roof rock. The ground control problem data served largely to define the relative degree of hazard associated with lineaments and the types of hazards encountered in the mine. Resistivity data helped confirm the presence of fractures where they were expected to occur both from surface geology and from the lineament data.

Jan 1, 1986